Question: $A=\left[\begin{array}{rr}13 & 1 & 17 & 5 & -16 \\2 & 6 & 3 & -1 & 8 \\0 &1 & -8 & -7 & -16 \\-2 &-13 &-15 & 15 & 5 \\6 &6 &1 & 2 & 1\end{array}\right]$ $A_{2,4}=$
Solution: Background An $m\times n$ matrix has $m$ rows and $n$ columns. $A=\left[\begin{array}{rr}A_{1,1} & \cdots & A_{1,n} \\\\\vdots \ & \ddots & \vdots \\\\A_{m,1} &\cdots &A_{m,n}\end{array}\right]$ Therefore, the entry $A_{{c},{d}}$ is located on row ${c}$ and column ${d}$. Finding $A_{2,4}$ $A_{{2},{4}}$ is located on row ${2}$ of $A$ : $\left[\begin{array}{rr}13 & 1 & 17 & 5 & -16 \\ {2} & {6} & {3} & {-1} & {8} \\0 &1 & -8 & -7 & -16 \\-2 &-13 &-15 & 15 & 5 \\6 &6 &1 & 2 & 1\end{array}\right]$ $A_{{2},{4}}$ is also located on column ${4}$ of $A$. $\left[\begin{array}{rr}13 & 1 & 17 & 5 & -16 \\ {2} & {6} & {3} & {\text{-1}} & {8} \\0 &1 & -8 & {-7} & -16 \\-2 &-13 &-15 & {15} & 5 \\6 &6 &1 & 2 & 1\end{array}\right]$ Therefore, $A_{{2},{4}}={-1}$. Summary $A_{2,4}=-1$